The present invention relates to apertured and unapertured structures as applied to at least one light emitting surface of semiconductor electroluminescent devices, such as, injection lasers or light emitting diodes (LEDs).
Heretofore, the suggestion has been made to apply multilayered structures to at least one facet surface of an electroluminescent device to provide a high level of reflectivity. If the reflectivity level is high at one of the facet surfaces, the current threshold of the device is reduced because more of the optical radiation is reflected back into optical radiation cavity of the device. Such a multilayered reflector structures are disclosed in U.S. Pat. Nos. 4,092,659 and 4,147,409.
The drawback in the employment of these types of reflectors is that, although reflectivity at one facet surface may be substantially increased, lowering current thresholds, mode stabilization is not achieved but rather undesirably enhanced toward unstabilization.
Furthermore, conventional gain-guided semiconductor injection lasers with known current confinement means exhibit nonlinearities in the light output power verses laser current characteristics. Abrupt changes in these characteristics are referred to as "kinks". Kinks can occur for contact stripe lasers at lower levels of a few milliwatts with contact stripe widths on the order of 8 um or wider.
The presence of kinks is now understood to result from spatial hole burning in the injected carrier profile which, in turn, causes the laser mode to shift laterally along the p-n junction of the injection laser. In the absence of a built-in mode guiding mechanism, such as, for example, curved or angled stripe geometry or a channeled substrate, the lateral mode shift produces a kink in the optical power output.
By providing a spatially selective reflector on at least one facet surface of the device, albeit an injection laser or LED, a higher central reflective region can be created at the point of light emission from the device to thereby increase mode stability and, as a result, increase the power level at which these output kinks occur.
What is proposed here is a multilayered reflector which, by means of an ablative process, will produce a reflector structure that has relatively high modal reflectivity at the center of optical radiation emission from the device, which reflectivity decreases radially from the center of such emission.
The employment of multilayered ablatable structures on at least one facet of such devices is not new in the art. U.S. Pat. No. 3,843,401 discloses an ablatable multilayered structure for determining the location of the p-n junction of a solid state laser device and causing the optical radiation to be emitted from a restricted region at the junction of the device.